linux-kernel - Re: [PATCH] sched/core: Schedule new worker even if PI-blocked

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Date:   Tue, 20 Aug 2019 15:50:14 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        tglx@...utronix.de
Subject: Re: [PATCH] sched/core: Schedule new worker even if PI-blocked

On Fri, Aug 16, 2019 at 06:06:26PM +0200, Sebastian Andrzej Siewior wrote:
> If a task is PI-blocked (blocking on sleeping spinlock) then we don't want to
> schedule a new kworker if we schedule out due to lock contention because !RT
> does not do that as well.

 s/as well/either/

> A spinning spinlock disables preemption and a worker
> does not schedule out on lock contention (but spin).

I'm not much liking this; it means that rt_mutex and mutex have
different behaviour, and there are 'normal' rt_mutex users in the tree.

> On RT the RW-semaphore implementation uses an rtmutex so
> tsk_is_pi_blocked() will return true if a task blocks on it. In this case we
> will now start a new worker

I'm confused, by bailing out early it does _NOT_ start a new worker; or
am I reading it wrong?

> which may deadlock if one worker is waiting on
> progress from another worker.

> Since a RW-semaphore starts a new worker on !RT, we should do the same on RT.
> 
> XFS is able to trigger this deadlock.
> 
> Allow to schedule new worker if the current worker is PI-blocked.

Which contradicts earlier parts of this changelog.

> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
>  kernel/sched/core.c |    5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3945,7 +3945,7 @@ void __noreturn do_task_dead(void)
>  
>  static inline void sched_submit_work(struct task_struct *tsk)
>  {
> -	if (!tsk->state || tsk_is_pi_blocked(tsk))
> +	if (!tsk->state)
>  		return;
>  
>  	/*
> @@ -3961,6 +3961,9 @@ static inline void sched_submit_work(str
>  		preempt_enable_no_resched();
>  	}
>  
> +	if (tsk_is_pi_blocked(tsk))
> +		return;
> +
>  	/*
>  	 * If we are going to sleep and we have plugged IO queued,
>  	 * make sure to submit it to avoid deadlocks.

What do we need that clause for? Why is pi_blocked special _at_all_?